The present invention relates to a method for desorbing fission iodine from a solution containing nitric acid and nuclear fuel.
The retention of radioactive iodine and its compounds is a safety problem in nuclear plants. The radioactive iodine is a volatile fission product of nuclear reactor fuels and is retained in the fuel pellets from the light water reactor fuel elements. When reprocessing a fuel element, iodine.sup.129 is of radiological significance, if cooling periods of greater than one year are involved since, due to its long half-life of 1.57.times.10.sup.7 years, iodine.sup.129 can accumulate in the biosphere.
Even when special measures are not taken, a large amount of the fission iodine will still be desorbed or separated into the dissolver off-gas during the dissolution of the nuclear fuel due to the evolution of water vapor-saturated nitric oxides, and possibly, stirring air. The residual non-distillable and hard to distill iodine remaining in the fuel solution is then distributed during subsequent processing operations among a plurality of different process streams and must, therefore, be removed from the vessel off-gas in the processing and waste treatment systems.
It is known to remove or drive the fission iodine from the fuel solution by distilling part of the nuclear fuel solution, thus producing a distillate, and/or by desorbing the fission product iodine from the heated fuel solution with the aid of larger amounts of stirring gas.
These prior art methods suffer from several disadvantages. For example, in order to assure release of the fission iodine from the fuel solution, except for the remainder of a few parts per thousand, up to approximately 50% of the fuel solution must be distilled and processed. Further, substantially greater gas throughputs are necessary in a downstream condenser (cocurrent flow of gas and condensate) with the direct return of the distillate into the dissolver. Finally, the resulting dilution of the contaminants contained in the dissolver off-gas requires considerably greater expenditures for equipment and filters.